1. Field of the Invention
This invention relates to polyolefin foams and films exhibiting enhanced antistatic properties, that is, foams and films which rapidly dissipate electrostatic charges and exhibit a reduced tendency to accumulate electrostatic charges. The polyolefin foams may be open or closed cell and are produced by known extrusion methods with foaming caused by known physical or chemical blowing agents. This invention also relates to the antistatic composition which causes the enhanced antistatic properties and the method of making the enhanced antistatic polyolefin foam.
2. Description of Related Art
Polyolefin foams, like almost all other synthetic polymeric materials, tend to acquire and accumulate electrostatic charges. For many applications it is desirable, if not essential, to have a foam material which either does not acquire an electrostatic charge, or which dissipates it rapidly.
Accumulation of electrostatic changes on all types of polymeric materials has been a long-standing problem, and a variety of techniques have been proposed to alleviate the problem. For example, compounds which migrate to the surface of the plastic or fiber have been incorporated in the composition to modify its electrical properties. Antistatic resins have been copolymerized with the base polymer in an effort to provide improved properties. Other antistatic compounds, such as quaternary (quatenary) ammonium salts, have been applied topically, i.e., by impregnation, or incorporated directly into the polymeric materials to provide a finished, or semi-finished product with improved antistatic properties.
There are several types of chemical antistatic compounds generally available. These include: cationic compounds, such as long-chain (those generally having about 4-22 carbon atoms) quaternary ammonium, phosphonium or sulfonium salts with, for example, chloride counter ions; anionic compounds, such as alkali salts or alkyl sulphonic, phosphinic, dithiocarbamic, or carboxylic acids; and nonionic compounds, such as ethoxylated fatty amines, fatty acid esters, ethanolamides, polyethylene glycol-esters, polyethylene glycol-ethers, and mono- and di-glycerides.
For example, U.S. Pat. No. 3,117,113 with a priority filing date in 1957, discloses the use, in combination with PVC (polyvinyl chloride resin), of quaternary ammonium compounds of the general class found useful in this invention. U.S. Pat. Nos. 3,335,123 and 3,407,187 disclose the use of quaternary ammonium compounds which are physically incorporated into polyolefins, PVC and other polymers, as by milling.
It is also known that certain quaternary ammonium salts can be added during the manufacture of polyurethane foam to impart improved antistatic properties to the cured foam. U.S. Pat. No. 3,933,697 discloses specific quaternary ammonium salts that can be incorporated as an ingredient into the composition prior to commencement of the foam-forming reaction; alternatively, it is suggested that the same compounds can be applied by impregnation of the finished urethane foam. U.S. Pat. No. 4,605,684 discloses the introduction of an antistatic additive composition prior to polymer formation of specific quaternary ammonium compounds and specific plasticizer compounds into a reaction mixture to form an antistatic polyurethane foam.
Polyolefin foam can be provided with antistatic properties by one of two methods: (1) extrusion production of an antistatic foam that has uniform properties throughout its volume and cross-section; and (2) post-treatment methods in which an antistatic composition is impregnated on the surface and, to the extent possible, throughout the interior structure of the foam product. The latter technique is the less preferable since it requires repeated handling and treatment of material thereby increasing its cost; it provides less uniform properties, which properties may indeed be only superficial if the thickness of the foam material is substantial; and the impregnant is subject to removal by wear and tear if the impregnated product is used over again. If used for packing sensitive electronic components, such as computer chips, the impregnated composition may attach to, and damage the article. Of course, in some applications such as laundry softeners or antistatic agents, it is intended that the impregnated foam lose its antistatic coating as it is transferred to the damp laundry during use in the dryer. However, in most applications it is desirable for the foam product to exhibit both permanent and uniform antistatic properties.
While the advantages of incorporating an additive by the extrusion method which will render the finished product antistatic are obvious, the selection of appropriate materials must be based upon their compatibility in the foam making process, and on their effect on the physical appearance and characteristics of finished foam product. The extrusion and expansion of the foam-forming ingredients must provide a uniform cell structure, the avoidance of splits, cracks and other defects, and finally a self-sustaining and stable extruded foam structure. The presence of even small quantities of additional compounds can have a markedly adverse effect on the extrusion and expansion mechanism of the finished product. Those familiar with the art will know of the adverse effects of low levels of contaminants or impurities which may be barely detectable.
Catonic compounds, such as quaternary ammonium salts, are known to affect the thermal stability of certain plastics, such as styrenics and hard PVC, when incorporated into those plastics.
It is therefore essential in seeking an agent or additive which will render the finished polyolefin foam antistatic, to find a compound or composition that is compatible with the foam making process. Other factors which must be taken into account are (1) the ability to mechanically incorporate the antistatic additive into the foam-forming composition using conventional equipment; and (2) the cost of the additive at its effective level or within its effective range and its impact on the final cost of the foam.